DESCENDINa THORACIC AORTA. 419 



(f) ThejmJmar interosseous arteries, usually three in number, but very liable to 

 variation, lie in front of the iiiterosseous spaces, supply the muscles there and 

 anastomose at the clefts of the fingers with the digital branches from the super- 

 ficial arch. 



It is by an enlargement of these small vessels that the deep palmar arch some- 

 times supphes the corresponding digital arteries in the absence of those usually 

 derived from the superficial arch. 



Varieties of the Arteries of the Hand. — The arteries of the hand frequently 

 vaiy from then- usual mode of distribution. 



(«7) By far the lai-ger number of deviations consist of a deficiency in either the 

 radial or ulnar system of arteries, accompanied by a corresponding increase ia 

 the other ; and it may be observed that the defect is much more commonly on 

 the part of the superficial, and the increase on the part of the deep set. 



(7y) In the second and smaller class of variations a deficiency in one or other 

 of the two systems is supplied, either by the enlargement of branches which 

 descend in front of the limb, as the superficial volar (from the radial), or the 

 median artery (from the anterior interosseous, as shown in fig. 273), or by the 

 enlargement of a metacarpal branch (from the radial) on the back of the hand. 



In illustration of these general remarks, the following modes of arrano-ement 

 of the vessels may be mentioned : — 



In the greater number of cases the superficial palmar arch is diminished, and 

 gives off fewer digital branches than usual. Generally only one branch is wanting 

 ^-iz., that which supplies the adjacent sides of the fore and middle fingers ; but 

 sometimes two or three branches are absent, or even all four, as when the ulnar 

 artery, after giving branches to the short muscles of the little finger, ends in the 

 deep palmar arch. In the last-mentioned case, which is rare, it is obvious that 

 the superficial arch is altogether wanting. 



These various deficiencies in the superficial palmar arch and its branches are 

 iisually compensated for by an enlargement of the deep arch, the palmar inter- 

 osseous branches of which, being increased in size, divide at the clefts of the 

 fingers, and form such collateral digital branches as are not derived from the 

 usual source. But a defective superficial arch may. as before mentioned, be 

 reinforced from other vessels, viz., from the superficial volar, from an enlaro-ed 

 median artery, or from a large metacarpal branch. 



It sometimes, hwt more rarely, happens, that the radial system of vessels is 

 deficient ; in which case the superficial arch (which belongs to the ulmar 

 system) may supply all the digital arteries to the thumb and fingers, or one of 

 these may be derived from the superficial volar, the median, or the radial 

 interosseous artery. 



DESCE]SrDII«"G THORACIC AORTA. 



From the point at which its arch is considered to terminate— the 

 lower margin of the third dorsal vertebra, the aorta descends along the 

 fore part of the spine to the fourth lumbar vertebra, where it divides 

 into the common iliac arteries. The direction of this part of the vessel 

 is not vertical, for as it follows the bend of the spine, upon which it 

 rests, it is necessarily concave forwards in the dorsal region, and convex 

 forwards in the lumbar. Again, as its commencement is at the left 

 side of the bodies of the vertebrse, and its termination also inclined 

 a little to the left, whilst about the last dorsal vertebra the vessel is 

 nearly in the middle line, there is produced another slight curve, 

 the convexity of which is to the right side. Within the thorax, where 

 the offsets are small, the aorta diminishes only slightly in size ; in the 

 abdomen the diminution is considerable, in consequence of large 

 branches being furnished to the viscera of that cavity. 



That part of the descending aorta which is situated in the thorax, is 



